This invention relates to power line carrier (PLC) communication systems and more particularly to a method and means of improving communication between the system units on a conventional power line.
A typical PLC system in a building utilizes the existing power distribution system in the building as transmission lines. The usual power distribution system in a building is a systems of wires, electric outlet receptacles, fuses and/or circuit breakers, switches and controls and permanently wired fixtures and appliances, installed for the purpose of distributing low frequency, 50 or 60 Hertz (Hz) alternating current (AC) power in the building. For a PLC system, this existing power distribution system provides the electrical path for non-power related radio frequency (RF) carrier transmissions modulated in some manner in order to transmit voice and/or data individually or in combination from one location in the building to another. These carrier signals are transmitted throughout the building via the existing power distribution system simultaneously with the transmission of the low frequency power.
In a PLC system, a transmitter generates modulated RF carrier signals at one location in the building, which are coupled to the existing power distribution system via an appropriate coupling network, usually at an electric outlet receptacle at that location, and a receiver at another location receives and demodulates the RF carrier providing the desired transmission of voice and/or data signals from the one location to the other.
In the United States, the usual source of electric power is the local Electric Power Company which generates and/or distributes three phase, 60 Hz alternating current (AC) power and supplies each subscriber electric power at one, two or three phases on two to four power wires of a power line to the subscriber's premises. Within the building, the subscriber usually provides the power distribution system. In the usual dwelling that may be a house, apartment, condominium, etc., and in small business and industrial buildings, the Power Company supplies two phases of 60 Hz on three wires: a first hot wire at phase one, a second hot wire at phase two and a neutral wire. Usually, the voltage across each of the hot wires and neutral is 110 volts AC (VAC) and across the two hot wires is 220 VAC. Also, usually, an effort is made when installing the power distribution system to supply part of the premises with 110 VAC at phase one, part with 110 VAC at phase two and a few locations in the premises with 220 VAC in such a way that the likely loads in the premises are evenly balanced on phases one and two. Also, usually in a dwelling and in small buildings, there are no power transformers in the distribution system.
Large electric power subscribers (usually business or industrial) may be provided by the Power Company at their building (or premises) power line voltage greater than 110 VAC at two or three phases. For those subscribers the power distribution system must include transformers in order to feed conventional 110 VAC outlets, fixtures and appliances in the building.
Clearly, PLC systems depend on the power wiring of the power distribution system in a building to perform as transmission lines to convey RF signals between units plugged into the power wiring. A transmission line is most efficient when it appears as a constant impedance at every point along its path. This is not the case with the usual power wiring in a premises, since the usual power wiring layout for a premises is a random distribution of branches from a common distribution panel and each branch has devices of widely varying impedances connected at random points. Furthermore, even though power wiring has standardized on three wire (H, N and G wires) power lines for 110 VAC during the past few decades, devices are often connected to the system by connection to only two of the three wires.
At present, in order for a PLC system to transmit and receive information effectively at all desired locations in the subscriber's building the transmitted power launched into the subscriber's power distribution system must often be so great that the RF radiation therefrom exceeds the power permitted by the Federal Communications Commission (FCC) regulations. An alternative to this has been to transmit lower RF power so as not to exceed FCC regulations and install special AC power line coupling devices throughout the building at distribution panels and at transformers and phase separation and protection networks. The results of these efforts are not entirely satisfactory and in many cases the only way to make the PLC system effective is to transmit greater RF power, and so exceed the FCC regulations.
PLC systems also have problems overcoming severe noise and signal losses associated with the power distribution system of the premises. The most severe signal attenuations are caused by devices plugged into the power line between two PLC units, as those devices shunt the PLC system signals and thus reduce the signal transmission between the units. For PLC system signals to overcome the noise levels existing on power lines, it is essential that the RF signals he received at a level that overrides the noise at the receiver. The governmental rules and regulations against excessive radiation of radio frequency signals sets an effective upper limit on the amount of power that can be used by a carrier current transmitter, thus making it necessary to use means to minimize losses and transfer as strong a signal as possible.
The usual power distribution system in a small premises, such as a dwelling, supports many services that connect only to the hot and neutral power wires (H and N) for their power, the ground wire (G) being reserved for safety connection to exposed conducting surfaces. PLC systems that use the hot and neutral wires for communication suffer from signal loss due to the shunting effect of other devices connected to these same two wires of the power distribution system. Several PLC systems have used the neutral and ground leads as the transmission conductors to attempt to overcome this shunting effect. This is not always effective since there is no assurance of consistency in wiring installations. In some cases, ground and neutral wires (G and N wires) are connected together in more than one place in the premises. Most often the G and N wires are connected together in the distribution system junction box. Obviously, shorted conductors will not effectively transfer radio signals along their path.